This invention relates in general to the collection of solar energy, and more particularly to arrays of panels for heating air by trapping heat from the sun under sheets of glass or other substantially transparent sheet material. Heated air may then be circulated to warm occupied spaces in buildings, for drying purposes, or stored for later use.
The growing awareness of fossil fuel shortages has sparked renewed interest in the possibility of economically capturing solar energy for heating purposes. Solar heating devices are not totally new, but previously have found little use because of the abundance and low cost of high density fossil fuels. While solar fuel is free, its low energy density requires large collection areas for most applications. For example, 1500 square feet of collection area facing the sun are typically required to fully heat an average home. Consequently, collection equipment costs are high, and most known solar heating systems cannot be justified economically.
Flat plate air heaters are known. The American Society of Heating, Radiation, and Airconditioning Engineers (ASHRAE) shows several designs in a recent chapter on Solar Heating and Cooling of Buildings. The simplest design is a single pane of glass admitting solar energy into a cavity where the incident radiation strikes a black absorbing surface which gets warm. Air moving through the cavity either by natural convection or forced air transfers the collected heat as desired. This inexpensive collector has several shortcomings: first, with just a single pane of glass, the collector loses considerable heat back to a cold outdoor environment, and second, the flat back plate does not provide enough area for effective heat transfer to the moving air stream. To overcome these problems, some solar air heaters have at least two transparent surface sheets, with an airspace therebetween, to minimize collector heat loss. Some solar air heaters have multiple absorbing sheets to increase the area for heat transfer to the moving airstream.
Solar air heaters are typically placed directly on a building surface, either a wall or the roof, facing the sun, with thermal insulation placed between the collectors and the space within the building. Since a large surface area is usually needed for collection, arrays of identical panels are usually used, the panels being fabricated in a size convenient for handling and also for minimizing waste of materials. Currently fabricated panels range in size from 2 feet by 4 feet up to 4 feet by 8 feet. The glass surface of the collector may serve as the weather surface of the building, but otherwise the collector panel is usually structurally redundant. While a plurality of air heating collector arrays have been installed between building framing members to reduce overall system costs, such designs have not been very versatile in permitting variable ductwork locations for air inlets and outlets to the panels.
An additional problem with flat plate heaters concerns their warm weather performance. Economical equipment is not yet available for using the sun's heat to drive absorption cycle airconditioning equipment, so it is necessary to reject captured or incident solar energy during the summer. Heat will build up rapidly in the air heating panel when air is not flowing, and some of the heat will be transferred through the insulation to indoor spaces, which is obviously an undesirable situation in the summer. Overheating can also damage construction materials and possibly even create a danger of a fire. Current techniques for preventing overheating include shading the collectors and ventilating the air cavities to the outdoors. Both of these approaches increase system costs.
Flat plate collectors (large glass-surfaced panels no more than 6-7 inches thick) are generally known for use as solar energy collectors. Concentrating collectors are capable of generating higher temperatures than flat plate collectors, but the latter reach adequate temperatures for space heating, are less expensive to build, and need not "track" the sun. Either air or a liquid (usually water) may be used to transfer heat from a flat plate collector to a storage medium or heated space. While water is generally a more effective heat transfer medium than air, solar water heaters are more expensive to build than solar air heaters since the liquid must be contained in a flow network in close contact with an absorbing surface. Also, the water heater faces maintenance problems with respect to leakage and freeze damage. Costs and reliability currently favor the air heater for space heating applications.
U.S. Pat. No. 3,863,621 discloses a solar wall system which includes two transparent glass or plastic sheets behind which is mounted one or more collector plates for absorption of the sun's rays, and the heat absorbed by the collector plate is then transferred to air passing in the space behind the collector plate. That patent discloses one embodiment wherein the absorptive collector plate is a louvered plate which presents more of its surface area to the sun's rays for absorption of solar energy.
U.S. Pat. No. 2,544,474 shows a solar water heater with water flow through a parallel array of flattened tubes similar to louvers; the entire array rotates to maintain the flattened surface perpendicular to the solar rays and the tube surfaces are flat black to absorb solar energy. As absorbers, the flattened tubes also radiate considerable energy back through the glass, reducing collector efficiency.
Solar rays strike the earth at a lower angle in winter than in summer. U.S. Pat. No. 2,625,930 shows a roof top collector system designed to admit low angle radiation into a collection area, yet reflect high angle radiation from the roof surface. U.S. Pat. No. 2,918,709, utilizes louvered slats in a window which may be oriented to admit radiation or reversed to reflect radiation. But there is no structure in the prior art which utilizes a selectively reflective and admissive geometry within a solar energy collector cavity.